It depends on how you created your maps in MapTool. The simplest way is to export the entire map as a single image. This locks everything into place and produces a single image file you can drag and drop into the map layer in Roll20. If you have lots of objects like tables, doors, chests, and such that you want to be able to move later... you'll need to remove all those from the map in MapTool, export the base map as a single image and then drag and drop that into Roll20. After that, you'll have to drag and drop and re-place every object you want to be able to move later.